U.S. patent application number 15/980707 was filed with the patent office on 2018-11-22 for optical device.
The applicant listed for this patent is Finisar Sweden AB. Invention is credited to David Adams, Efthymios Rouvalis, Jan-Olof Wesstrom, Martin Zirngibl.
Application Number | 20180335681 15/980707 |
Document ID | / |
Family ID | 62217971 |
Filed Date | 2018-11-22 |
United States Patent
Application |
20180335681 |
Kind Code |
A1 |
Adams; David ; et
al. |
November 22, 2018 |
OPTICAL DEVICE
Abstract
An optical interference modulator comprises a main input, a main
output, an optical splitter connected to the main input, first and
second MMI couplers, each with a first primary-end access port
connected to the splitter; a second primary-end access port
connected to the main output; a first secondary-end access port
connected to a respective primary wave-guide; and a second
secondary-end access port connected to a respective secondary
wave-guide. A light reflector is arranged to reflect light incident
from said primary and secondary waveguides back into the same
respective waveguide such that light travelling through the
respective waveguide from the respective secondary-end access port,
after reflection, travels back to the same secondary-end access
port. For the MMI couplers, at least one of the respective primary
and secondary waveguides comprises a respective light phase
modulating device arranged to modulate the phase of light
travelling along the corresponding waveguide in either
direction.
Inventors: |
Adams; David; (Stockholm,
SE) ; Rouvalis; Efthymios; (Berlin, DE) ;
Wesstrom; Jan-Olof; (Stockholm, SE) ; Zirngibl;
Martin; (Middletown, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Finisar Sweden AB |
Jarfalla |
|
SE |
|
|
Family ID: |
62217971 |
Appl. No.: |
15/980707 |
Filed: |
May 15, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62507284 |
May 17, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/225 20130101;
H04B 10/0795 20130101; G02F 2001/217 20130101; H04B 10/5053
20130101; G02F 2001/212 20130101 |
International
Class: |
G02F 1/225 20060101
G02F001/225; H04B 10/079 20060101 H04B010/079 |
Claims
1. Optical interference modulator, comprising a main input for
light to be modulated, a main output for modulated light, an
optical splitter connected to the main input, respective first and
second MMI (MultiMode Interference) couplers, each with a
respective first primary-end access port connected to the splitter;
a respective second primary-end access port connected to the main
output; a respective first secondary-end access port connected to a
respective primary waveguide; and a respective second secondary-end
access port connected to a respective secondary waveguide, wherein
the modulator further comprises a light reflector arranged to
reflect light incident from said primary and secondary waveguides
back into the same respective waveguide, so that light travelling
through the waveguide in question from said respective first or
second MMI coupler secondary-end access port will, after
reflection, travel back to the same first or second MMI coupler
secondary-end access port, wherein, for both the first and second
MMI couplers, at least one of the said respective primary and
secondary waveguides comprises a respective light phase modulating
device arranged to modulate the phase of light travelling along the
waveguide in question in both directions; and wherein both the
first and second MMI couplers are arranged so that a different
respective phase shift is imparted to light travelling between a
primary-end access port and a secondary-end access port in a cross
state as compared to in a bar state.
2. Optical interference modulator according to claim 1, wherein,
for each MMI coupler, at least one of said respective primary and
secondary waveguides comprises a respective fixed phase shift
device, the combination of fixed phase shift devices being selected
so that destructive interference results between light exiting the
respective first primary-end access ports of each MMI coupler and
being combined in the said splitter, at least when the said phase
modulating device is set so as to modulate the light output through
the output waveguide according to at least two different
predetermined modulation symbols.
3. Optical interference modulator according to claim 1, wherein,
for at least one MMI coupler, a light phase shifting device is
arranged between the splitter and the first primary-side access
port and arranged to impart a phase shift of .alpha., and another
light phase shifting device arranged between the second
primary-side access port and a combining device arranged to combine
the light from the second primary-side access port and the second
primary-side access port of the other MMI coupler and arranged to
impart a phase shift of 2.pi.-.alpha., where .alpha. is a constant
such that 0.ltoreq..alpha.<2.pi..
4. Optical interference modulator according to claim 1, wherein the
modulator is a Mach-Zehnder modulator, and wherein each of the said
first and second MMI couplers are arranged to split light incident
from the main input into different respective waveguides and also
to combine reflected light incident from such different
waveguides.
5. Optical interference modulator according to claim 4, wherein the
first MMI coupler forms both the splitter and combiner of a first
optical interferometer arranged to control the real part of the
electromagnetic field of the light to be modulated, wherein the
second MMI coupler forms both the splitter and combiner of a second
optical interferometer arranged to control the imaginary part of
the electromagnetic field of the light to be modulated.
6. Optical interference modulator according to claim 1, wherein the
first and second MMI couplers are arranged to impart a relative
phase shift of .pi./2 between cross state transmitted light passing
the MMI coupler in question and bar state light passing the MMI
coupler in question, respectively.
7. Optical interference modulator according to claim 1, wherein the
modulator is arranged to convey reflected light exiting said second
primary-end access ports to different respective secondary-end
access ports of a third MMI coupler, which third MMI coupler is
arranged to output such reflected light via a first primary-end
access port of the third MMI to the main output.
8. Optical interference modulator according to claim 7, wherein the
modulator comprises at least one SOA (Semiconductor Optical
Amplifier), arranged to amplify reflected light from the first and
second MMI couplers to the third MMI coupler and/or downstream of
the third MMI coupler.
9. Optical interference modulator according to claim 8, wherein
said third MMI coupler also comprises a second primary-end access
port, and wherein the modulator is arranged to convey reflected
light from said second primary-end access port of the third MMI
coupler to a light detector comprised in the modulator, or to
provide that light on an output port.
10. Optical interference modulator according to claim 1, wherein
the modulator is arranged to direct light passing through the
reflector to a light detector.
11. Optical interference modulator according to claim 1, wherein
the main input, the main output, the first and second MMI couplers
are arranged in one and the same plane, and wherein the modulator
is arranged to convey reflected light exiting the said respective
second primary-end access ports of the first and second MMI
couplers to the main output via a waveguide crossing with respect
to a waveguide arranged to convey light incident from the main
input to the first and second MMI couplers.
12. Optical interference modulator according to claim 1, wherein
the main output, the first and second MMI couplers are arranged in
one and the same plane, and wherein the modulator is arranged to
convey reflected light exiting the said respective second
primary-end access ports of the first and second MMI couplers to
the main output via a waveguide extending past and around a
waveguide arranged to convey light from the main input to the said
first primary-end access ports.
13. Optical interference modulator according to claim 12, wherein
the modulator comprises a semiconductor laser, which is integrated
as a part of the same optical chip as the first and second MMI
couplers and arranged to provide light to be modulated to the main
input, and wherein the said laser is arranged in the same plane as
the first and second MMI couplers and between said waveguides
arranged to convey light from the second primary-end ports to the
main output, alternatively that the laser is arranged to supply
light to be modulated to a light coupling device arranged to direct
incident light, not arriving in the same plane as the said
waveguides, to the first and second MMI couplers, and that the said
coupling device is arranged between a first and a second waveguide
arranged to convey light from the respective second primary-end
access ports to the main output.
14. Optical interference modulator according to claim 1, wherein
the light reflector is a cleaved facet of an integrated optical
chip comprising the first and second MMI couplers, provided with a
high reflectivity coating, or where the arms terminate with a
grating reflector or a TIR type reflector, or with an etched,
non-cleaved facet that is then coated with a high reflectivity
coating.
15. Optical interference modulator according to claim 1, wherein
the light phase modulating device comprises four parallel phase
modulating electrodes, each arranged to individually apply a
respective voltage to a respective primary or secondary waveguide,
which phase modulating electrodes are arranged to achieve
modulation of the output light by variable phase modulation as a
function of said electrical voltage.
16. Optical interference modulator according to claim 15, wherein
respective electrical contacts for providing said voltage are
connected to the respective modulating electrodes via respective
electrical connectors, wherein at least two of said connectors
cross a plane of reflection associated with the light
reflector.
17. Optical interference modulator according to claim 15, wherein
respective electrical connectors for providing said voltage are
connected to the respective modulating electrodes, which connectors
are lumped element electrodes, alternatively that the connectors
are arranged with a transmission line that is impedance matched to
a RF driver circuitry providing said voltage.
18. Aggregated optical interference modulator comprising two
modulators according to claim 1, wherein the aggregated modulator
comprises a main aggregated modulator input, a main aggregated
modulator output, an optical aggregated modulator light splitter
and an optical aggregated modulator light combiner, which
aggregated modulator splitter is arranged to split light incident
from said main aggregated modulator input to each main input of the
respective modulators, and which aggregated modulator combiner is
arranged to combine light incident from the main outputs of the
respective modulators
19. Aggregated optical interference modulator according to claim
18, wherein the said aggregated modulator combiner comprises a
polarization rotation device arranged to rotate the polarization of
the phase light that is incident from at least one of the modulator
main outputs before combining such light with light incident from
the other modulator main output to form the light signal output via
the main aggregated modulator output, or where the combining of the
two dissimilar polarizations is done off the chip.
20. Modulated light outputting device comprising a laser and an
optical interference modulator according to claim 1, wherein the
control device is arranged to control the optical modulator, via a
set of applied time-varying control voltages to respective
electrodes comprised in the said interference modulator, so as to
modulate light output from the laser forming an output modulated
optical signal.
21. Modulated light outputting device according to claim 20,
wherein the modulation is a phase shift keying or binary phase
shift keying modulation.
22. Modulated light outputting device according to claim 21,
wherein the modulation is performed by applying said control
voltages in a push-pull fashion to pairs of control electrodes,
wherein each such pair of control electrodes is arranged along a
respective pair of primary and secondary waveguides.
23. Modulated light outputting device according to claim 22,
wherein the voltage applied to each electrode results in a relative
phase shift of less than .pi./4 for light passing the electrically
affected material once.
24. Method for modulating an optical signal using an optical
interference modulator, which optical interference modulator
comprises a main input for light to be modulated, a main output for
modulated light, an optical splitter connected to the main input,
respective first and second MMI (MultiMode Interference) couplers,
each with a respective first primary-end access port connected to
the splitter; a respective second primary-end access port connected
to the main output; a respective first secondary-end access port
connected to a respective primary waveguide; and a respective
second secondary-end access port connected to a respective
secondary waveguide, wherein the modulator further comprises a
light reflector arranged to reflect light incident from said
primary and secondary waveguides back into the same respective
waveguide, so that light travelling through the waveguide in
question from said respective first or second MMI coupler
secondary-end access port will, after reflection, travel back to
the same first or second MMI coupler secondary-end access port,
wherein, for both the first and second MMI couplers, at least one
of the said respective primary and secondary waveguides comprises a
respective light phase modulating device arranged to modulate the
phase of light travelling along the waveguide in question in both
directions; and wherein both the first and second MMI couplers are
arranged so that a different respective phase shift is imparted to
light travelling between a primary-end access port and a
secondary-end access port in a cross state as compared to in a bar
state, which method comprises the steps of a) selecting a
modulation scheme covering at least two different modulation
symbols; b) for the said light phase modulating device along the
said primary and secondary waveguides, selecting respective fixed
light phase shifts so that destructive interference results for
each of said modulated symbols, and c) modulating the said symbols
by selecting different variable light phase shifts for said light
phase modulating device along the said primary and secondary
waveguides.
25. Method according to claim 24, wherein the said modulation of
said symbols is performed in a push-pull fashion wherein the
modulation depth is not complete.
26. Method according to claim 25, wherein the said modulation is a
phase shift keying or binary phase shift keying modulation.
27. Method for monitoring a modulated optical signal using an
optical interference modulator, which optical interference
modulator comprises a main input for light to be modulated, a main
output for modulated light, an optical splitter connected to the
main input, respective first and second optical couplers, each with
a respective first primary-end access port connected to the
splitter; a respective second primary-end access port connected to
the main output; a respective first secondary-end access port
connected to a respective primary waveguide; and a respective
second secondary-end access port connected to a respective
secondary waveguide, wherein the modulator further comprises a
light reflector arranged to reflect light incident from said
primary and secondary waveguides back into the same respective
waveguide, so that light travelling through the waveguide in
question from said respective first or second secondary-end access
port will, after reflection, travel back to the same first or
second secondary-end access port, wherein, for both the first and
second couplers, at least one of the said respective primary and
secondary waveguides comprises a respective light phase modulating
device arranged to modulate the phase of light travelling along the
waveguide in question in both directions; wherein reflected light
exiting from a particular primary-end access port is conveyed, via
a waveguide, to a second output, and which method comprises the
steps of a) providing a light detector, arranged to detect said
light conveyed to said second output; b) the light detector
detecting an amplitude and/or a phase of the detected light and
reporting the measurement value to a control unit performing the
monitoring.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of and priority to U.S.
Provisional Application No. 62/507,284 filed May 17, 2017. The
62/507,284 application is incorporated herein by reference.
FIELD
[0002] The embodiments herein relate to an optical device, in
particular to an optical modulator. The embodiments herein also
relate to a method for modulating an optical signal and a method
for monitoring a modulated optical signal.
BACKGROUND
[0003] Optical modulators are known in the art. In many
applications, in particular for high speed optical communication
systems, a modulated light wave is used to carry digital
information from a sender to a receiver. In many such systems, the
modulation may be phase and/or amplitude modulation. Examples
include binary amplitude modulation with a return-to-zero (RZ) or
non-return-to-zero (NRZ) optical pulse stream format, and phase
shift keying modulation techniques, such as Binary Phase Shift
Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), and
Quadrature Amplitude Modulation (QAM) techniques, such as QAM8,
QAM16 and QAM64. In each of these communication formats, the
modulated light wave will carry information about one or several
symbols selected among a predetermined set of symbols.
[0004] In order to achieve such modulation of a carrier light wave,
it is known to split the carrier light wave using a splitter, and
to recombine the carrier light wave in a combiner after a relative
phase shift of the different light paths between the splitter and
combiner, forming a Mach-Zehnder interferometer. The phase shift
can for instance be achieved using electrodes attached to one or
more of said paths, to each of which electrodes a variable electric
signal can be applied so that the refractive index of the path
waveguide material changes. Such variable phase shift can be
combined with a predetermined fixed phase shift for each waveguide.
This way, each symbol can be modulated as a unique combination of
total phase shifts along each path.
[0005] Some modulators can comprise a plurality of so-called
"child" interference modulators that are arranged in parallel, and
that are comprised within a larger "parent" interference modulator.
A modulator in which a first parallel-coupled MZM controls the
imaginary part of the electromagnetic field (Q value) and a second
parallel-coupled MZM controls the corresponding real part (I value)
is called an IQ modulator (IQM).
[0006] WO 2011022308 A2 discloses using a Mach-Zehnder modulator
(MZM), yielding two paths, or two parallel-coupled child MZMs each
on one respective path of a parent MZM, yielding in total four
paths, with variable-voltage electrodes on each path, for such
modulation.
[0007] A problem with such interferometric light wave phase
modulators is that these modulators are typically very large when
compared with the wavelength of the light in the optical
waveguides. This is attributable not only to the typically weak
refractive index modulation response of the waveguide material in
response to applied voltage, but also to the additional device
length that is required to accommodate the optical splitters and
combiners and their input and output access waveguides. The
relatively large device dimensions have an impact on the cost of
the modulator device itself, and the chip size will also impact the
size and cost of the modulator submount and of the optical module
package enclosure.
[0008] Another problem associated with the typically weak
refractive index response of the waveguide material to reverse
voltage bias is that either lengthy phase modulation electrodes are
required, and/or high modulation voltage swings are required, to
attain a desired dynamic phase modulation amplitude or intensity
extinction ratio under device operation. A problem with lengthy
electrodes on the phase modulator arms is that these contribute a
parasitic capacitance that limits the maximum achievable modulation
bandwidth of the modulator. The use of travelling wave electrodes
partly mitigates the bandwidth limitation that is imposed by the
capacitance on the arms, but these travelling wave electrodes are
typically longer, and they are more complex to manufacture, than
the basic "lumped" electrical circuit element phase modulation
electrodes.
[0009] An additional problem with the use of large amplitude
dynamic voltage swings is that the electrical power dissipation is
approximately proportional to the square of the modulation voltage
swing amplitude, so that a large voltage swing will contribute
substantially to the energy cost of the operation of the
communication system.
BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS
[0010] Some embodiments herein may solve one or more of the above
described problems.
[0011] Hence, an example embodiment relates to an optical
interference modulator, comprising a main input for light to be
modulated, a main output for modulated light, an optical splitter
connected to the main input, respective first and second MMI
(MultiMode Interference) couplers, each with a respective first
primary-end access port connected to the splitter; a respective
second primary-end access port connected to the main output; a
respective first secondary-end access port connected to a
respective primary waveguide; and a respective second secondary-end
access port connected to a respective secondary waveguide, wherein
the modulator further comprises a light reflector arranged to
reflect light incident from said primary and secondary waveguides
back into the same respective waveguide, so that light travelling
through the waveguide in question from said respective first or
second MMI coupler secondary-end access port will, after
reflection, travel back to the same first or second MMI
secondary-end access port, wherein, for both the first and second
MMI couplers, at least one of the said respective primary and
secondary waveguides comprises a respective light phase modulating
device arranged to modulate the phase of light travelling along the
waveguide in question in both directions; and wherein both the
first and second MMI couplers are arranged so that a different
respective phase shift is imparted to light travelling between a
primary-end access port and a secondary-end access port in a cross
state as compared to in a bar state.
[0012] Furthermore, an example embodiment relates to a method for
modulating an optical signal using an optical interference
modulator, which optical interference modulator comprises a main
input for light to be modulated, a main output for modulated light,
an optical splitter connected to the main input, respective first
and second MMI (MultiMode Interference) couplers, each with a
respective first primary-end access port connected to the splitter;
a respective second primary-end access port connected to the main
output; a respective first secondary-end access port connected to a
respective primary waveguide; and a respective second secondary-end
access port connected to a respective secondary waveguide, wherein
the modulator further comprises a light reflector arranged to
reflect light incident from said primary and secondary waveguides
back into the same respective waveguide, so that light travelling
through the waveguide in question from said respective first or
second MMI coupler secondary-end access port will, after
reflection, travel back to the same first or second MMI
secondary-end access port, wherein, for both the first and second
MMI couplers, at least one of the said respective primary and
secondary waveguides comprises a respective light phase modulating
device arranged to modulate the phase of light travelling along the
waveguide in question in both directions; and wherein both the
first and second MMI couplers are arranged so that a different
respective phase shift is imparted to light travelling between a
primary-end access port and a secondary-end access port in a cross
state as compared to in a bar state, which method comprises the
steps of a) selecting a modulation scheme covering at least two
different modulation symbols; b) for the said light phase
modulating device along the said primary and secondary waveguides,
selecting respective fixed light phase shifts so that destructive
interference results for each of said modulated symbols, and c)
modulating the said symbols by selecting different variable light
phase shifts for said light phase modulating device along the said
primary and secondary waveguides.
[0013] Also, an example embodiment relates to a method for
monitoring a modulated optical signal using an optical interference
modulator, which optical interference modulator comprises a main
input for light to be modulated, a main output for modulated light,
an optical splitter connected to the main input, respective first
and second optical couplers, each with a respective first
primary-end access port connected to the splitter; a respective
second primary-end access port connected to the main output; a
respective first secondary-end access port connected to a
respective primary waveguide; and a respective second secondary-end
access port connected to a respective secondary waveguide, wherein
the modulator further comprises a light reflector arranged to
reflect light incident from said primary and secondary waveguides
back into the same respective waveguide, so that light travelling
through the waveguide in question from said respective first or
second coupler secondary-end access port will, after reflection,
travel back to the same first or second secondary-end access port,
wherein, for both the first and second couplers, at least one of
the said respective primary and secondary waveguides comprises a
respective light phase modulating dew vice arranged to modulate the
phase of light travelling along the waveguide in question in both
directions; wherein reflected light exiting from a particular
primary-end MMI coupler access port is conveyed, via a waveguide,
to a second output, and which method comprises the steps of a)
providing a light detector, arranged to detect said light conveyed
to said second output; b) the light detector detecting an amplitude
and/or a phase of the detected light and reporting the measurement
value to a control unit performing the monitoring.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the following, the invention will be described in detail,
with reference to exemplifying embodiments of the invention and to
the enclosed drawings, wherein:
[0015] FIGS. 1a, 1b, 1c and 1d illustrate a conventional modulating
device, with corresponding phasor and constellation diagrams;
[0016] FIG. 2a shows a modulating device;
[0017] FIGS. 2b and 2c are enlarged detail views of respective MMI
couplers of FIG. 2a;
[0018] FIG. 3 illustrates an unfolded representation of the folded
modulating device shown in FIG. 2a;
[0019] FIG. 4a illustrates a first preferred embodiment of the
invention;
[0020] FIG. 4b illustrates a second preferred embodiment of the
invention;
[0021] FIG. 5a illustrates an unfolded representation of the folded
modulator shown in FIG. 4b;
[0022] FIG. 5b illustrates a phasor diagram that represents the
light signal that is reflected back to the laser for the modulator
in FIG. 5a;
[0023] FIG. 6 illustrates a third preferred embodiment of the
invention;
[0024] FIGS. 7a and 7b illustrate, in respective graphs, a fraction
of a transmitted symbol power that is reflected back to the laser
as a function of the modulation voltage depth, for modulators of
the types shown in FIGS. 2, 4a, 4b, and 6;
[0025] FIG. 8a illustrates a fourth preferred embodiment;
[0026] FIG. 8b illustrates a fifth preferred embodiment;
[0027] FIG. 9 shows a sixth preferred embodiment of the present
invention;
[0028] FIG. 10 is a flowchart illustrating a method according to a
first method aspect of the invention; and
[0029] FIG. 11 is a flowchart illustrating a method according to a
second method aspect of the invention.
[0030] All Figures are simplified, overview illustrations provided
for understanding of the principles proposed by the present
inventors, and are in general not to scale.
DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS
[0031] Using optical communication technology, it is possible to
send information using both amplitude and phase modulation schemes.
One of the advantages of this, as compared to only using amplitude
modulation, is that more information can be sent over the same
optical frequency band.
[0032] One type of device that can transmit phase and amplitude
modulated signals is an IQ modulator (IQM), consisting of a
parallel pair of child Mach-Zehnder interferometers disposed along
each arm of a parent Mach-Zehnder interferometer. An example of a
conventional modulator 100 of this type is illustrated in FIG.
1a.
[0033] The modulator 100 comprises a main or parent Mach-Zehnder
Interferometer (MZI) 120 with a main input waveguide 101 and a main
output waveguide 102. The respective paths of the main MZI 120
comprise one respective child MZI 130, 140 each. The child MZIs
130, 140 are parallel-coupled, where a first child MZI 130
comprises first 131 and second 132 paths of the modulator 100 and a
second child MZI 140 comprises third 141 and fourth 142 paths of
the modulator 100.
[0034] One example of a conventional advanced modulation format
that can be impressed onto an optical carrier wave by such an IQ
modulator is the Quadrature Phase Shift Keying (QPSK) scheme,
involving four distinct symbols 191, 192, 193, 194 as shown in FIG.
1d in a phasor diagram of conventional type. The circle in FIG. 1d
illustrates the unity amplitude transmission.
[0035] In QPSK modulation, the four symbols 191-194 differ in both
the real part (the I-axis) and the imaginary part (the Q-axis) of
the complex E-field, as illustrated in FIG. 1d. FIGS. 1b and 1c
illustrate the I+, I-, and Q+, Q- arm phasor positions in the
absence of applied modulation voltage (labelled Io+, Io-, and Qo+,
Qo-) and their direction of rotation in push pull modulation, with
the sum net positive or negative I and Q vector amplitudes along
the real and imaginary output field axes.
[0036] The convention used herein for the construction and
interpretation of phasor diagrams, is that the propagation of light
corresponds to counter-clockwise movement of a phasor (positive
angular rotation). Furthermore, for the sake of consistency, it is
assumed that all launched optical beams begin with a phase of zero
where these encounter the first element that can affect the
relative phase of one optical path vs. one or more other paths.
Phase delays correspond to negative (clockwise) phasor
rotation.
[0037] It is understood that there may be several, different sets
of individual static phase shifts that result in a certain desired
net interference performance at a data output port, and that also
provide a desired net interference performance for any light
reflected back to a port that delivers light to the modulator from
a light source, as will be detailed below.
[0038] For reasons of simplicity, optical losses due to device
fabrication imperfections or the partial absorption of light
signals caused by the modulator material in response to applied
voltage or current tuning are neglected in the following
description. Symmetric splitting and combining will be assumed at
all the splitters and couplers in all of the diagrams. Even though
such symmetric splitting and combining is preferred, the
embodiments disclosed herein could also comprise asymmetric
splitting and/or combining for one or several of the described
splitters and combiners.
[0039] Hence, for the IQ modulator 100 illustrated in FIG. 1a, the
second child MZI 140 is associated with an initial static phase
shift 143 of +.pi./2 radians, which is applied before the light
enters the child MZI 140 in question, but not to light travelling
to the first child MZI 130. With such a setup, the first child MZI
130 will be effective for modulating the real part of the E-field
(I-axis in the phasor diagram of FIG. 1d), and the second child MZI
140 will be effective for modulating the imaginary part of the
E-field (Q-axis in FIG. 1d). By applying respective electrical
signals onto electrodes 131b, 132b; 141b, 142b, arranged along
respective parallel arms 131, 132, 141, 142 of the child MZI:s 130,
140, in a push-pull fashion, an I-data stream represented in the
modulated light wave output on waveguide 102, by the electrical
signal applied over time onto electrodes 131b, 132b of the child
MZI 130 can be represented as variations of the real part of the
E-field of the modulated light wave, and correspondingly a Q-data
stream represented by the electrical signal applied over time onto
electrodes 141b, 142b of the child MZI 140 can be represented in
the modulated light wave output on waveguide 102 as variations of
the corresponding imaginary part of the E-field. In QPSK, this can
be done in a straightforward way by sending, in the form of
corresponding electrical signals, the I-data stream to the I-child
130 and the Q-data stream to the Q-child 140.
[0040] The result is illustrated in FIG. 1d, where the vectors 195,
196, 197, 198 correspond to the light that that has travelled
through paths I+, I-, Q+ and Q-, respectively, and for a certain
symbol where I+=1, I-=-1, Q+=1 and Q-=-1, so that the variable
phase modulation applied by the respective electrodes 131b, 132b,
141b, 142b is +.pi./2, -.pi./2, +.pi./2 and -.pi./2, respectively.
Hence, the total phase shift, including the static modulation, for
each path is according to Table I for this particular symbol:
TABLE-US-00001 TABLE I Vector Data Total phase shift 195 I+ = 1 ( -
.pi. 2 ) + .pi. 2 = 0 ##EQU00001## 196 I- = -1 .pi. 2 + ( - .pi. 2
) = 0 ##EQU00002## 197 Q+ = 1 .pi. 2 + ( - .pi. 2 ) + .pi. 2 = .pi.
2 ##EQU00003## 198 Q- = -1 .pi. 2 + .pi. 2 + ( - .pi. 2 ) = .pi. 2
##EQU00004##
[0041] The resulting vector 199 represents the output light wave
output on waveguide 102 after final combination of the light waves
output by the two child MZIs 130, 140.
[0042] The modulation according to the invention generally takes
place in a corresponding fashion, with differences that will become
clear from the following.
[0043] FIG. 2a illustrates an optical interference modulator 200.
The optical interference modulator 200 uses a reflective IQ
modulator geometry that can impress QPSK modulation onto an optical
carrier wave in a manner that is equivalent to that achieved with a
conventional IQM modulator, such as the one illustrated in FIG.
1a.
[0044] The optical interference modulator 200 comprises a main
input 201 for light to be modulated, a main output 202 for
modulated light (a data output port), an optical splitter 203 in
the form of a 2.times.2 MMI (MultiMode Interference) coupler
connected to the main input 201, and connecting, via waveguides,
the output ports of the splitter 203 (the MMI outputs) to
respective first 233 and second 243 MMI (MultiMode Interference)
couplers. In general, in the terminology used herein, such an MMI
coupler comprises two primary-side access ports and two
secondary-side access ports. As illustrated in FIG. 2b, the first
MMI coupler 233 comprises primary-side access ports 233a, 233b and
secondary-side access ports 233c, 233d. As illustrated in FIG. 2c,
the second MMI coupler 243 comprises primary-side access ports
243a, 243b and secondary-side access ports 243c, 243d. The term
"primary-side" refers to a side of the MMI in question facing
towards the main input 201 and/or output 202, and/or away from a
light reflector 260 as described below; the term "secondary-side"
refers to a side of the MMI in question facing away from the main
input 201 and/or output 202, and/or towards such a light reflector
260. It is realized that such an MMI coupler may have more than two
primary-side access ports, and more than two secondary-side access
ports.
[0045] In the case illustrated in FIG. 2a-2c, each of said MMI
couplers 233, 243 comprises a respective first primary-end access
port 233a; 243a connected to the said splitter 203. Furthermore,
each of said MMI couplers 233, 243 comprises a respective second
primary-end access port 233b; 243b that are not connected to the
main output 202, but to an "MPD" ("Monitor Photo Diode") port, used
for monitoring (see below). The respective first primary-side
access ports 233a, 243a deliver light that has been reflected at
260 (see below) to the splitter 203, whereby such reflected light
is combined in the splitter 203 and delivered on a second
primary-side access port of the splitter 203, which in turn is
connected to the main output 202. A corresponding first
primary-side access port of the splitter 203 is connected to the
main input 201 and the laser. It is realized that "connected to",
in this context, may refer to a direct or indirect optical
connection, where an indirect optical connection could for example
be an optical path through a lens train or a surface grating
coupler.
[0046] Moreover, each of said MMI couplers 233, 243 comprises a
respective first secondary-end access port 233c; 243c connected to
a respective primary waveguide 231; 241, and a respective second
secondary-end access port 233d; 243d connected to a respective
secondary waveguide 232; 242. Hence, the primary 231 and secondary
232 waveguides connected to the first MMI coupler 233 may form the
parallel-connected waveguides of a first child MZI 230 as described
above in connection to FIG. 1a, while the primary 241 and secondary
242 waveguides connected to the second MMI coupler 243 may form the
parallel-connected waveguides of a second child MZI 240 in the
corresponding manner. In this context, the first 230 and second 240
MZI:s may be arranged in a respective parallel-connected arm of a
parent MZI 220 comprised in the modulator 200.
[0047] The modulator 200 further comprises a light reflector 260
arranged to reflect light incident from said primary 231, 241 and
secondary 232, 242 waveguides back into the same respective
waveguide, so that light travelling through the waveguide in
question from said respective first 233 or second 243 MMI coupler
secondary-end access port 233c, 233d; 243c, 243d will, after
reflection, travel back to the same first or second MMI
secondary-end access port. In other words, instead of the child
MZI:s 230, 240 having a respective separate combiner, the
respective MMI coupler 233, 243, acting as the splitter in the
child MZI 230, 240 in question, also acts as combiner in the child
MZI in question, after the light has been reflected at the light
reflector 260.
[0048] Furthermore, for both the first 233 and second 243 MMI
couplers, at least one of the said respective primary 231, 241 and
secondary 232, 242 waveguides comprises a respective light phase
modulating means 231a, 232a, 241a, 242a, 231b, 232b, 241b, 242b,
arranged to modulate the phase of light travelling along the
waveguide 231, 232, 241, 242 in question in either direction (both
directions). In particular, it is preferred that at least one of
said primary 231, 242 or secondary 232; 242 waveguides of both the
first 230 and second 240 child MZI:s comprises such a respective
light phase modulating means. Preferably, the light phase
modulating means referred to herein is at least a variable
(dynamic, controllable) light phase modulating means 231b, 232b,
241b, 242b, which may impart different light phase modulating
magnitudes as a response to an electric signal applied to the light
phase modulating means in question.
[0049] Furthermore, both the first 233 and second 243 MMI couplers
are arranged so that a different respective phase shift is imparted
to light travelling between a primary-end access port 233a, 233b;
243a, 243b and a secondary-end access port 233c, 233d; 243c, 243d
in a cross state as compared to in a bar state. It is realized that
examples of "cross state" light paths comprise 233a.fwdarw.233d and
233b.fwdarw.233c; while examples of "bar state" light paths
comprise 233a.fwdarw.233c and 233b.fwdarw.233d. Such difference
between a cross state and a bar state transmission is equally true
regarding all 2.times.2 MMI couplers disclosed herein. For other
types of MMIs, such as an N.times.M MMI, where N,M>1 and at
least one of N,M>2, corresponding light path differences may
exist as between said cross and bar states (but in general having
different light path length differences than in a 2.times.2 MMI,
depending on entry/exit port combination), and may be used in a
corresponding way as described herein for the special case
N=M=2.
[0050] In particular, it is preferred that the first 233 and second
243 MMI couplers are arranged to impart a relative phase shift of
.pi./2 radians between cross state transmitted light passing the
MMI coupler 233, 243 in question and bar state light passing the
MMI coupler 233, 243 in question, respectively.
[0051] Due to its folded geometry, such an optical modulator device
200 in general has more compact device dimensions as compared to a
non-folded device, and can simultaneously, due to the use of MMI
couplers 233, 243 with different phase shifts between cross and bar
states, be designed to achieve very low optical reflections back to
the optical input port 201 of the modulator 200. Moreover, it is
possible to achieve a reduction in the required modulation voltage
swing when compared with the prior art, as well as an increase in
the modulation bandwidth when compared with prior art
modulators.
[0052] Furthermore, the folded geometry of the modulator 200
enables a reduction in the length of phase modulation arms.
[0053] The phase shifting means 231a, 232a, 241a, 242a are
preferably static or fixed, while phase shifting means 231b, 232b,
241b, 242b are dynamic or variable. Herein, "static" or "fixed"
phase shifting means may be controllable so as to reach a
particular calibration or configuration of the modulator 200, but
their imparted phase shifts are generally not changed with each
modulated symbol. In contrast thereto, a "dynamic" or "variable"
phase shifting means is arranged to impart a phase shift which is
different for different modulated symbols. Preferably, each child
MZ 230 240 arm 131, 132, 141, 142 comprises at least a respective
variable phase shifting means 131b, 132b, 141b, 142b, and
preferably at least one or two, preferably all four, of the arms
131, 132, 141, 142 also comprise a respective fixed phase shifting
means 131a, 132a, 141a, 142a.
[0054] The double pass of the optical carrier wave through any
static phase modulation means 231a, 232a, 241a, 242a leads to a
reduction in the required length or of the required applied current
or voltage bias for the desired static fixed phase modulation in
question, when compared with a non-folded modulator layout. The
corresponding is also true regarding the variable phase shift means
231b 232b, 241b, 242b.
[0055] Moreover, the size of the modulator 200 can also be reduced
for a folded geometry, when compared with a conventional modulator
layout, not only because of a possible reduction in the static or
dynamic phase electrode lengths, but also because of the double
pass through the said optical coupler elements 233, 243, which
serve as splitters in the initial pass and later serve as combiners
when the light is returning through the modulator 200 after the
reflection from the interferometer arm terminations.
[0056] In the following, several different detailed ways of
achieving these and other advantages will be presented, in
combination with a number of exemplifying embodiments of the
present invention.
[0057] According to a preferred embodiment, for each MMI coupler
233, 243, at least one of said respective primary 231; 241 and
secondary 232; 242 waveguides comprises a respective fixed phase
shift means 231a, 232a, 241a, 242a. Preferably, such fixed phase
shift means 231a, 232a, 241a, 242a are used in combination with
said variable phase shift means 231b, 232b, 241b, 242b along the
same respective waveguide 231, 232, 241, 242. Then, the combination
of fixed phase shift means 231a, 232a, 241a, 242a is selected, in
particular preferably together with a particular selected variable
phase shift means 231b, 232b, 241b, 242b control program, so that a
desired interference pattern results between light exiting the
respective first primary-end access ports 233a; 243a of each MMI
coupler 233, 243, being combined in the said splitter 203 and
delivered back to the main input 201 and the laser. Such desired
destructive interference pattern may be specifically accomplished
by such selection of fixed phase shifts when the said variable
phase modulating means 231b, 232b, 241b, 242b are controlled and
set so as to modulate the light output through the main output
waveguide 202 according to at least two different predetermined
modulation symbols.
[0058] Such fixed phase shifts may be selected, in relation to a
used variable phase shift modulating program, in different ways. A
number of examples will be provided in the following.
[0059] Specifically, one of the child MZI:s 230, 240, such as the
first child MZI 230, may be associated with an initial static phase
shift 234, such as of +.pi./2 radians, which is applied before the
light enters the child MZI 130 in question but not to light
travelling to the other child MZI 140.
[0060] In general, the variable phase shifts of the variable phase
shifting means 231b, 232b, 241b, 242b are controlled by a control
device 250, which may in turn be controlled using a control unit
(not shown in the Figures).
[0061] Preferably, and as described above, the modulator 200 is a
Mach-Zehnder modulator, wherein each of the said first 233 and
second 243 MMI couplers are arranged to split light incident from
the main input 201 into different respective waveguides 231, 232;
241, 242, and also to combine reflected light incident from such
different waveguides.
[0062] In particular, it is preferred that the first MMI coupler
233 forms both the splitter and combiner of a first optical
interferometer 230 arranged to control the real part of the
electromagnetic field of the light to be modulated, and that the
second MMI coupler 243 forms both the splitter and combiner of a
second optical interferometer 240 arranged to control the imaginary
part of the electromagnetic field of the light to be modulated.
[0063] According to a preferred embodiment, the light reflector 260
is a cleaved facet of an integrated optical chip comprising the
first 233 and second 243 MMI couplers, provided with a high
reflectivity coating. Alternatively, the arms 231, 232, 241, 242
may terminate with a grating reflector or a TIR type reflector, or
comprise an etched, non-cleaved facet that is then coated with a
high reflectivity coating.
[0064] Hence, in FIG. 2a, the four interferometer arms 231, 232,
241, 242 each terminate at such a facet 260, preferably having a
high reflectivity coating applied to said facet, such that incident
optical beams are retro-reflected at this facet 260, and thereby
traverse the phase modulation 231, 232, 241, 242 arms a second
time, in the opposite direction. Alternatively, other reflective
elements 260, such as gratings, can be utilized to achieve the
retro-reflection at the termination of the interferometer arms 231,
232, 241, 242.
[0065] Further with reference to FIG. 2a, the Y junction splitter
elements of FIG. 1a have been replaced by respective 2.times.2 port
optical MMI couplers 233, 234, and this also applies to the
splitter 203, which is preferably also an MMI coupler, preferably a
2.times.2 MMI coupler. The first 2.times.2 MMI coupler 203 after
the laser, which laser delivers light to the main input waveguide
201, serves to split the light into the two child Mach-Zehnders
230, 240 for the forward pass through the arms 231, 232, 241 242,
and this 2.times.2 MMI coupler 203 also serves to combine the
reflected beams from the two child Mach-Zehnders 230, 240 into the
data output port 202 after the return pass. Likewise, the 2.times.2
MMI couplers 233, 243 at the start of each child Mach-Zehnder 230,
240 serve to split the input beam into the respective
interferometer arms 231, 232, 241, 242, and also to recombine these
beams after the return pass through the interferometer arms 231,
232, 241, 242. Preferably, the 2.times.2 MMI couplers 233, 243 at
the start of each child Mach-Zehnders 230, 240 may comprise one
port (namely, the respective second primary-side access port 233b,
243b), arranged to direct light into an integrated or external
monitor photodiode (MPD--see FIG. 2a). This enables, via a feedback
mechanism based upon readings of such an MPD and performed by a
control unit of the above mentioned type, accurate device
configuration for transmission, and/or to maintain a proper device
configuration over its operational lifetime. Optionally, the
2.times.2 MMI couplers 233, 243 at the start of one or each of the
child Mach-Zehnders 230, 240 may be replaced by more compact
1.times.2 MMI splitters, but this makes it less convenient to
collect a signal that can be directed to MPD's as illustrated in
FIG. 2a.
[0066] As is illustrated in FIG. 2a, the light phase modulating
means comprises four parallel phase modulating electrodes 231b,
232b, 241b, 242b, each arranged to individually apply a respective
voltage to a respective primary 231; 241 or secondary 232; 242
waveguide. Moreover, the variable phase modulating electrodes 231b,
232b, 241b, 242b in question are arranged to achieve modulation of
the output light by variable phase modulation as a function of said
electrical voltage.
[0067] According to one preferred embodiment, the modulator 200
comprises respective electrical contacts, for providing said
voltage to said respective variable phase shifting means 231b,
232b, 241b, 242b, which contacts are connected to the respective
modulating electrodes via respective electrical connectors, wherein
at least two, preferably all four, of said connectors cross a plane
of reflection associated with the light reflector 260. This is
illustrated in FIG. 2a, where the connectors run from the control
device 250 to each individual variable electrode affecting the
variable phase shift of the respective dynamic phase shifting means
231b, 232b, 241b, 242b.
[0068] FIG. 3 is an alternative representation of the basic
reflective IQ modulator 200 design illustrated in FIG. 2a, but
where the device 200 layout has been conceptually "unfolded" about
the reflective termination plane 260, such that the optical
transmission can be visualized to occur from left to right in the
Figure. FIG. 3 shares the same reference numerals with FIG. 2a. In
this representation, the -.pi./2 relative phase shift (a phase
delay, caused by increased optical path length) that occurs for the
diagonal path (cross-state) transmission within the 2.times.2 MMI
couplers 233, 243 (relative to propagation to the output port on
the non-diagonal, bar-state path) has been included.
[0069] In the following Table II, the static phase shift values
that are needed to recover the same four point constellation
diagram that is given in FIG. 1d are provided.
TABLE-US-00002 TABLE II Fixed phase shift Amount a 0 b 0 c +.pi./2
d +.pi./2 .epsilon. +.pi.
[0070] As stated above, FIGS. 1b and 1c show the phasor diagrams
for the FIG. 1a configuration. However, FIGS. 1b and 1c are also
applicable to FIG. 3, showing the phasor diagrams for the I+, I-,
and Q-, Q- modulator arms, in their relative positions when no bias
is applied (vectors having a o subscript), and where the curved
arrows within the dashed ellipses show how the phasors rotate
toward each other in push pull operation, to form either a net
positive or negative output vector along the I axis (FIG. 1b) or Q
axis (FIG. 1c). It is noted that the scheme used is a push-pull
scheme, in the sense that the same relative phase shift I+ is
applied in the variable phase shifting means 232b as the relative
phase shift I- applied in the means 231b, but with opposite signs,
and correspondingly regarding the relative phase shifting means
241b, 242b, applying Q+ and Q-. It is also noted that the light
power back to the laser is substantially zero in the absence of
applied modulation (the modulator is in the "off" state), and that
under active operation the light reflected back to the laser is
substantially equal to the amplitude of the data signal that
arrives at the output port for this modulator configuration. Each
such symbol is defined as a particular combination of I and Q, for
instance according to the following scheme:
TABLE-US-00003 TABLE III Symbol I+ I- Q+ Q- 1 1 -1 1 -1 2 -1 1 1 -1
3 1 -1 -1 1 4 -1 1 -1 1
[0071] Some of the key benefits of a folded IQM geometry are that
the required dynamic modulation voltage swing amplitudes and/or the
interferometer phase arm electrode 231a, 232a, 241a, 242a, 231b,
232b, 241b, 242b lengths can be substantially reduced when compared
with a conventional, non-folded, IQ modulator. Furthermore, the
magnitude of the applied voltage or current that might be required
to set or to fine-tune the fixed phase adjust sections 234, 231a,
232a, 241ba, 242a (sections a, b, c, d, or .epsilon.) can also be
reduced when compared with the conventional IQM that is illustrated
in, for instance, FIG. 1a. The reduced physical dimensions and
reduced modulation voltage and/or reduced tuning current amplitudes
and/or reduced tuning section lengths together enable a substantial
reduction of the folded geometry modulator device fabrication cost,
and/or a reduction of the power dissipation during operation.
[0072] As shown in FIG. 2a, the reflective plane 260 at one
extremity of the modulator 200 provides an opportunity to place RF
(Radio Frequency alternating current) or DC (Direct Current) driver
circuitry adjacent to this termination plane, in addition to the
usual freedoms to place RF or DC drive circuitry along the faces of
the modulator 200 that are parallel to the light propagation
direction. This is preferred. Alternatively, an optical detector
array or other optical apparatus can be situated adjacent to the
highly reflective modulator termination plane 260, to monitor the
relative phases or amplitudes of the fractions of light that are
transmitted from one or more of the interferometer arms 231, 232,
241, 242 through their respective high reflection termination 260.
The information collected by such monitors could be utilized for
modulator operational configuration, or for source laser channel
selection, and/or to monitor transmitter performance over life.
Preferably, information measured by such a monitor is provided, in
a feedback loop, to the control unit mentioned above, to be used in
the control of the modulator 200.
[0073] Hence, the modulator 200 is preferably arranged to direct
light passing through the reflector 260 to a light detector. The
light detector may be arranged externally to the modulator 200,
while the reflector 260 of the modulator 200 is arranged to let
through a certain proportion, such as between 2% and 10%,
preferably about 5%, of the light incident to the reflector 260
from each arm waveguide 231, 232, 241, 242, so that such
non-reflected light can enter into and be detected by such an
external detector.
[0074] Alternatively, the reflector 260 at the end of each arm 231,
232, 241, 242 can be arranged as an integrated grating within each
arm 231, 232, 241, 242, and in that case the light detectors can be
arranged as one or several integrated parts within the modulator
200 chip itself.
[0075] The gratings or high reflectivity facet coatings that are
utilized as the reflector 260 can optionally be designed to
strongly reflect the carrier wave signals, while permitting
substantial (such as at least 50%) transmission at other optical
wavelengths, for example to reduce transmitter system noise arising
from laser and/or semiconductor optical amplifier spontaneous
emissions.
[0076] According to a preferred embodiment, respective electrical
connectors for providing the voltage to at least the variable phase
shifting means 231b, 232b, 241b, 242b are connected to respective
modulating electrodes. Such connectors are preferably lumped
element electrodes. Herein, the term "lumped element electrode"
refers to an electrode having a length that is 15% or less of the
electrical wavelength that corresponds to the symbol rate that is
delivered by the applied RF signal. Hence, the variable phase
electrode 231b, 232b, 241b, 242b length in question has been
sufficiently reduced that the parasitic capacitance is diminished
to the point that a desired modulation bandwidth will be attainable
with lumped element phase electrode construction, rather than a
more complex travelling wave electrode construction.
[0077] Such lumped element electrodes simplifies the manufacturing
process. Any reduction of the dynamic phase modulation electrode
231b, 232b, 241b, 242b length(s) will increase the maximum
achievable modulation bandwidth, when compared with a conventional
non-folded modulator layout, as a consequence of the reduction in
the parasitic capacitance of the phase modulation electrode for the
folded modulator layout.
[0078] Alternatively, such connectors may be arranged with a
transmission line that is impedance matched to a RF driver
circuitry providing said voltage.
[0079] As described above, in FIG. 2a, a third MMI coupler 203, in
addition to the first 233 and second 243 MMI couplers, is arranged
between the main input 201 and the first 233 and second 243 MMI
couplers. FIGS. 4a and 4b illustrate two preferred embodiments
according to the invention, in which a third MMI coupler 404 is
arranged not between a main input 401 and a first 433 and a second
443 MMI coupler, but between a main output 402 and said first 433
and second 443 MMI couplers. It is noted that, while the third MMI
coupler 203 of FIG. 2a is arranged between the first 233 and second
243 MMI couplers and both the main input 201 and output 202, the
third MMI coupler 404 of FIGS. 4a and 4b is arranged between the
first 433 and second 434 MMI couplers and only the main output 402.
FIGS. 4a and 4b share the same reference numerals for corresponding
parts with FIG. 2a, apart from the first digit in each reference
numeral.
[0080] In particular, in these preferred examples, the modulator
400 is arranged to convey reflected light exiting the second
primary-end access ports of the first 433 and second 443 MMI
couplers, respectively, to the main output 402. I other words, the
said respective second primary-end access ports of the first 433
and second 443 MMI couplers are connected to the main output
402.
[0081] More specifically, in the modulator 400, reflected light
exiting the second primary-end access ports of the first 433 and
second 443 MMI couplers, respectively, is conveyed to different
respective secondary-end access ports of the third MMI coupler 404,
which third MMI coupler 404 in turn is arranged to output such
reflected light via a first primary-end access port of the third
MMI 404 to the main output 402. It is noted that the terms
"primary-side" and "secondary-side" are used according to the
definitions given above, also for the third MMI coupler 404.
[0082] As is shown in FIGS. 4a and 4b, the third MMI coupler 404
preferably also comprises a second primary-end access port. Then,
the modulator 400 is arranged to convey reflected light from said
second primary-end access port of the third MMI coupler 404 to a
light detector MPD comprised in the modulator 400 (such as
integrated in the modulator 400), or to provide that light on a
separate output port, for provision of that light to an external
light detector. The light detected by such an internal or external
detector is preferably fed to the above discussed control unit, in
a feedback loop, so as to affect the control of the modulator
400.
[0083] It is specifically noted that no light is conveyed from the
third MMI coupler 404 back to the main input 401. It is of
particular importance to note that, unlike the configurations that
are illustrated in FIGS. 3 and 2a, the preferred embodiments that
are illustrated in FIGS. 4a and 4b can be configured to transmit
substantially zero light back to the modulator input port at all of
the at least two symbols in the data symbol set used.
[0084] FIGS. 4a and 4b illustrate two alternatively preferred
layouts of the module 400. In both FIGS. 4a and 4b, the main output
402, the first 433 and second 443 MMI couplers are arranged in one
and the same plane.
[0085] However, in FIG. 4a, the modulator 400 is arranged to convey
reflected light exiting the said respective second primary-end
access ports of the first 433 and second 443 MMI couplers to the
main output 402, via the third MMI coupler 404, via a waveguide
crossing 405 with respect to a waveguide 406 arranged to convey
light incident from the main input 401 to the first 433 and second
443 MMI couplers. This makes it simple to arrange also the main
input 401 in the same plane as the main output 402 and the first
433 and second 443 MMI couplers. Any path length imbalance between
the arms may be adjusted using a separate phase adjustment means
(not shown in FIG. 4a) in at least one of the arms, arranged to
impart an additional asymmetric (with respect to the two arms in
question) phase shift, preferably a static phase shift, so as to
configure the two waveguide paths back to the third MMI coupler 404
to balance any optical phase difference between the light that is
output from these two paths.
[0086] In contrast to FIG. 4a, in FIG. 4b, the modulator 400 is
arranged to convey reflected light exiting the said respective
second primary-end access ports of the first 433 and second 443 MMI
couplers to the main output 402 via a waveguide 407 extending past
and around a waveguide 408 arranged to convey light from the main
input 401 to the said first primary-end access ports of the first
433 and second 443 MMI couplers. With such a configuration, there
is no need for a correction of the nominal physical path length
difference between the two arms as described in connection to FIG.
4a.
[0087] If a laser source is integrated within the modulator 400
chip, or if the light is coupled vertically into the modulator 400
for example with a surface grating of slanted integrated mirror
element to capture input radiation from out of the modulator 400
plane, then one may use a symmetric geometry with equal final
combiner 404 arm path lengths. In this case, an additional
advantage is that no waveguide crossing is required.
[0088] In particular in the example illustrated in FIG. 4b, it is
hence preferred that the modulator 400 comprises a semiconductor
laser, which is integrated as a part of the same optical chip as
the first 433 and second 443 MMI couplers, and arranged to provide
light to be modulated to the main input 401. Then, the said laser
may be arranged in the same plane as the first 433 and second 443
MMI couplers and between the waveguides arranged to convey light
from the second primary-end ports of the first 433 and second 443
MMI couplers to the main output 402. Alternatively, the laser in
question may be arranged to supply light to be modulated to a light
coupling means ("port to couple in laser input" in FIG. 4b),
arranged to direct incident light, not arriving in the same plane
as the said waveguides, to the first 433 and second 443 MMI
couplers. In this latter case, it is preferred the said coupling
means is arranged between a first and a second waveguide arranged
to convey light from the respective second primary-end access ports
of the first 433 and second 443 MMI couplers to the main output
402, and in particular to the third MMI coupler 404 in case such a
third MMI coupler 404 is used. Hence, such light from the laser is
then incident at an angle to the paper as shown in FIG. 4b, and
redirected by the said redirecting or coupling means so as to be
parallel to the paper. It is noted that the redirecting or coupling
means is arranged in the same plane as the first 433, second 443
and third 404 MMI coupler in this case.
[0089] In general when using an externally arranged laser with the
present invention, an isolator may be placed between such an
external laser and the corresponding modulator input port, to
further diminish the impact on the laser performance of whatever
residual reflection returns to the modulator input port, according
to the particular modulator configuration and mode of dynamic
operation in question.
[0090] The embodiment illustrated in FIGS. 4a and 4b, with a third
MMI coupler 404 arranged to combine light exiting the second
primary-side access ports of the respective first 433 and second
443 MMI couplers, provides particular advantages, in particular in
applications where an incomplete phase modulation depth of the I
and Q arm voltages is permissible (in other words, where the
dynamic voltage swing on the I and Q phase electrodes is less than
the voltage required to cause a phase shift having a magnitude of
.pi./2 on each arm under push pull modulation). In such
applications, the length of the variable phase electrode 431b,
432b, 441b, 442b length can be reduced, for example to reduce RF
driver power consumption. In the case of incomplete modulation
depth, using the example of the four point constellation diagram
for the QPSK modulation format, the magnitude of the output signal
is reduced, but the relative positions of the four data points are
unchanged as compared to the constellation diagram shown in FIG.
1d.
[0091] The scenario of incomplete phase modulation depth can
facilitate the utilization of the basic reflective IQM modulator
200 geometry illustrated in FIG. 2a, because then the optical
signal that is reflected back to the optical input port will have
the same time-averaged magnitude as the output data signal.
Accordingly, any reduction in the output data signal amplitude will
also reduce the level of optical isolation that will be required
between the laser source and the modulator 200.
[0092] The exemplifying modulators 400 illustrated in FIGS. 4a and
4b, however, are preferred to the configuration shown in FIG. 2a,
in particular when the latter is operated with a reduced modulation
depth. In the FIGS. 4a and 4b embodiments, the power reflection
back to the laser is substantially zero at all QPSK symbols,
regardless of the modulation depth, and the worst case reflected
power amplitude during the transitions between symbols is
substantially lower than for the case of the reflective modulator
in FIG. 2a, where the relative advantage over the modulator in FIG.
2a improves as the modulation depth is reduced.
[0093] In FIGS. 4a and 4b, a simple Y-branch splitter is used
between the main input 401 and the first 433 and second 443 MMI
couplers. However, this splitter may also be an MMI coupler, such
as a 1.times.2 or a 2.times.2 MMI coupler.
[0094] FIG. 5a shows an "unfolded" representation of FIG. 4b, with
optical transmission from left to right, but where the static phase
shifting means 431a, 432a, 441a, 442a and the dynamic phase
shifting means 431b, 432b, 441b, 442b are represented as the total
added phase shift after the double pass (forward and reflected)
through each child MZ arm 431, 432, 41, 441, and through each phase
adjust section (unlike in FIG. 3a, where the forward and reverse
optical travel through the dynamic and static phase modulation
sections are represented by separate boxes).
TABLE-US-00004 TABLE IV Fixed phase shift Amount a +.pi./2 b
-.pi./2 c +.pi./2 d -.pi./2
[0095] The phasors I+, I-, Q+, and Q- for the modulator in FIG. 5a
add in a manner identical FIGS. 1b and 1c, when the phase shifts
that are listed in table IV are utilized in FIG. 5a.
[0096] FIG. 5b shows the phasor diagram for the light that is
reflected back to the main input port 401, i.e. to the laser, for
the same modulation scheme as illustrated in FIGS. 4a-5a, with push
pull modulation acting as in 1b-1c, and with the same static phase
shifts as presented in Table IV. As is clear from the phasor
diagram 5b, the total light reflected back to the laser (the
summation of the I and Q vectors from the two child MZ's) is
substantially zero, regardless of the push or pull polarity on
either child MZ arm pair.
[0097] Hence, the configuration according to FIGS. 4a and 4b may
achieve a drastic reduction in the time averaged optical reflected
signal amplitude back to the laser input port (the main input 401),
as compared to the amplitude of the output signal at the data port
(the main output 402), as compared with the configuration of FIG.
2a.
[0098] For an illustrative QPSK modulation case based on the
modulator in FIG. 4, if we designate the absolute magnitude phase
modulation depth on each child 430, 440 arm 431, 432, 441, 442 as
.PHI., then in the case of full modulation depth (assuming
push-pull modulation as is preferred), .PHI.=.pi./2, with
|.PHI.|<.pi./2 for the more general case of incomplete
modulation depth. Designating the total (static and dynamic) phase
modulation on the I arms 431, 432 as A and B, and the Q arms as C
and D, and with A=-B, and C=-D for push pull operation, then the
net amplitude of the real reflected field is sin A+sin B+sin C+sin
D=0, and the net imaginary field amplitude is cos A+cos B-cos C-cos
D=0, in other words giving a net field and intensity amplitude of
zero at the four data constellation points in question, regardless
of the phase modulation depth. By comparison, for a reflective
modulator such as that shown in FIG. 2a or 3a, the power
reflectance back to the laser at all of the 4 QPSK symbols is the
same as the output signal power level for all modulation depths.
The power reflectance of the reflective modulator embodiments
illustrated in FIGS. 2a and 3a, compared to the reflective
modulator embodiment of FIG. 4, as a function of the modulation
depth, are shown in FIG. 7a.
[0099] If we instead quantify the reflection performance as the
worst case optical reflection during the transients between symbol
transitions for the modulator in FIG. 4, then the result depends on
the phase modulation depth .PHI.. For equal power launched into
each child Mach Zehnder 430, 440, the power reflectance back to the
main input 401 is
R = ( 1 - cos .phi. ) 2 4 . ##EQU00005##
approaches zero as the phase modulation depth is reduced. The
maximum power reflectance level during the transients between
transmitted symbols for the FIGS. 2a and 3a embodiments, as well as
for the FIG. 4 embodiment, as a function of the modulation depth,
are shown in FIG. 7b.
[0100] In one aspect, the present invention relates to a modulated
light outputting device comprising the laser and an optical
interference modulator according to the invention of any of types
described herein, or an aggregated optical interference modulator
as described below. The control device 250 is arranged to control
the optical modulator in question, via a set of applied
time-varying control voltages to respective electrodes comprised in
the said interference modulator, so as to modulate light output
from the laser forming an output modulated optical signal. This has
all been described above.
[0101] However, in this case it is specifically preferred that the
modulated light outputting device in question is such that the
modulation is a quadrature phase shift keying or binary phase shift
keying modulation. Binary phase shift keying can for example be
achieved by modulating between diametrically opposed symbols in a
QPSK diagram. Specifically, it is preferred that the modulation in
question is performed by applying said control voltages in a
push-pull fashion, as explained above, to pairs of control
electrodes, wherein each such pair of control electrodes is
arranged along a respective pair of primary and secondary
waveguides of the above described type.
[0102] As mentioned, in one preferred embodiment, full modulation
depth is used. However, in an alternatively preferred embodiment,
which is particularly useful with the configurations illustrated in
FIGS. 4a and 4b, the voltage applied to each electrode results in a
relative phase shift of less than .pi./4 for light passing the
electrically affected material once. In other words, the modulation
depth is less than full.
[0103] FIG. 6 illustrates, in an "unfolded" manner, as in FIG. 5a,
another preferred embodiment of the invention in the form of a
modulator 600. FIG. 6 shares the same reference numerals with FIGS.
4a-5a for corresponding parts, apart from the first digit in each
reference numeral.
[0104] As shown in FIG. 6, for at least one of the said first 633
and second 643 MMI couplers, preferably for only one 643 of said
MMI couplers 633, 643, an additional static light phase shifting
means 671 is arranged between the splitter (the splitter between
the main input 601 and the first 633 and second 643 MMI couplers)
and the first primary-side access port of the MMI coupler 643 in
question. This additional static phase shifting means 671 is
arranged to impart a static phase shift of a to light passing
through the phase shifting means 671 in question. Furthermore, yet
another additional static light phase shifting means 672 is
arranged between the second primary-side access port of the same
MMI coupler 633 in question and the combining means 604 arranged to
combine the light from the second primary-side access port of the
MMI coupler 633 in question and the second primary-side access port
of the other MMI coupler 643. This other phase shifting means 672
is arranged to impart a phase shift of 2.pi.-.alpha.. .alpha. is
the same for means 671 and 672, and is a constant such that
0.ltoreq..alpha.<2.pi..
[0105] In general, .alpha.may be 0, in which case the configuration
illustrated in FIG. 6 simplifies to the one illustrated in FIG. 5a,
even not having phase shifting means 671, 672 at all. However, it
may also be the case that 0<.alpha.<2.pi.. In general in
these cases, the value of a is selected so as to minimize the light
power reflected back to the main input 601, as a function of
voltage modulation depth, for modulation schemes having amplitude
and phase modulation output symbol sets where it is in general not
possible to get a zero reflection for all symbols in the modulation
symbol system in question. In other words, these static phase
shifting elements 671 and 672 provide the freedom to minimize the
light reflection back to the laser at the symbols or during the
transitions between symbols within any particular complex
modulation symbol set, in a manner that is completely independent
of the phase relationship that must be established between the I
and Q vectors when these add at the data output port.
[0106] As is clear from FIGS. 7a and 7b, the present invention
provides very low reflections back to a main input in a folded
geometry for a wide range of applications.
[0107] FIGS. 8a and 8b illustrate further preferred embodiments of
the present invention, each being an optical interference modulator
aggregate 800, 80 comprising a respective combination of two
interconnected modulators of the above type. In general, such an
aggregate 800, 850 comprises two modulators 800a, 800b; 850a, 850b
of the above described type. The aggregated modulator 800, 850
furthermore comprises a main aggregated modulator input 800c; 850c
and an optical aggregated modulator light splitter 800d; 850d. The
aggregated modulator splitter 800d; 850d is arranged to split light
incident from the main aggregated modulator input 800; 850c to each
main input of the respective modulators 800a, 800b; 850a, 850b.
Preferably, these components 800a, 800b, 800c, 850d; 850a, 850b,
850c, 850d may be arranged as integrated parts on one and the same
chip 801; 851.
[0108] The light output from the respective main outputs of the two
modulators 800a, 800b; 850a, 850b may be handled in different ways.
Preferably, the aggregated modulator 800, 850 comprises a main
aggregated modulator output 800f; 850f as well as an optical
aggregated modulator light combiner 800e; 850e, which aggregated
modulator combiner 800e; 850e is arranged to combine light incident
from the main outputs of the respective modulators 800a, 800b;
850a, 850b and present such combined light at the said main
aggregated modulator output 800f; 850f.
[0109] The reflective modulator embodiments described herein
harness the advantages of compactness and reduced static and
dynamic voltage swings, as described earlier, while also disposing
a set of output waveguide paths that do not take the light output
signal back to the input port. The disposition of these separate
output waveguide paths facilitates the addition of important post
modulation functions, such as integrated polarization rotation, and
optionally an integrated polarization combiner.
[0110] According to a first alternative embodiment, an integrated
chip 851; 851 on which the aggregated modulator 800; 850 is
arranged comprises two different main outputs, from which output
light is fed into off-chip bulk components arranged to provide
polarization rotation and polarization combination of the output
light so as to produce a combined main output light beam.
[0111] According to a second alternative embodiment, the integrated
chip 851; 851 on which the aggregated modulator 800; 850 is
arranged also comprises two different main outputs, where one or
both main outputs is or are arranged with polarization rotation
functionality on the chip 801; 851. Polarization rotation may be
performed on-chip or off-chip.
[0112] According to a third alternative embodiment, the integrated
chip 801; 851 on which the aggregated modulator 800; 850 is
arranged comprises only one main output, where all of the required
polarization rotation and polarization combining are performed on
the chip 801; 851 using suitable, integrated optical
components.
[0113] In both FIGS. 8a and 8b, the respective main input 800c;
850c and main output port 800f; 850f are arranged on opposing sides
of the said modulator chip 801; 851. The east and west faces where
the respective child MZ arms (of the modulators 800a, 800b; 850a,
850b) terminate have high reflectivity coatings ("HR Coating"), and
the main input 800c; 850c and output 800f; 850f ports can be
implemented as surface grating couplers. Alternatively, the input
800c; 850c and output 800f; 850f faces may have conventional input
and output edge couplers and AR (Anti Reflection) facet coatings,
and the east and west reflective child MZ arm terminations might be
integrated reflectors, such as gratings or etched TIR (Total
Internal Reflection) mirrors.
[0114] In FIG. 8a, the modulators 800a, 800b are of the type
illustrated in FIG. 2a, while the modulators 850a, 850b of FIG. 8b
are of the type illustrated in FIG. 4a. It is realized that such an
aggregated modulator can be implemented using any combination of at
least two individual modulators according to any one exemplifying
embodiment described herein.
[0115] In general, the aggregated modulator combiner 800; 850
preferably comprises a polarization rotation means arranged to
rotate the phase light incident from one of the modulator 800a,
800b; 850a, 850b main outputs before combining such light with
light incident from the other modulator main output 800a, 800b;
850a, 850b to form the light signal output via the main aggregated
modulator output 800; 850f.
[0116] According to one preferred embodiment, which is illustrated
in FIG. 9 (sharing reference numerals with FIG. 4a, apart from the
initial digits--"9" and "4", respectively--in each reference
numeral), the modulator 900 comprises at least one SOA
(Semiconductor Optical Amplifier) 909, arranged to amplify
reflected light from the first 933 and second 943 MI couplers to
the output 902, in particular between the first 933 and second 943
MMI couplers and the third MMI coupler 904 and/or downstream of the
third MMI coupler 904.
[0117] As seen in FIG. 9, a respective SOA 909 is included on each
child MZ 930, 940 output waveguide, to optionally and possibly
dynamically controllably boost the respective output signal power
during conventional operation, and/or to attenuate the respective
output signal under reverse bias of the SOA 909 in question during
a reconfiguration of the source laser, such as a change in the
lasing wavelength. Preferably, each such SOA 909 is individually
connected to, and controlled by, the above described control
unit.
[0118] According to one preferred alternative, illustrated in FIG.
9, both the respective child MZ 930, 940 output arms are arranged
with their own separate SOA 909. According to an alternative
embodiment, however, one single SOA can be arranged before the
final output 902, after the third and final MMI coupler 904.
[0119] In addition to the modulator as such, the invention also
relates to a method for controlling such a device, and in
particular for modulating an optical signal using an optical
interference modulator of the types described herein.
[0120] Such a method is illustrated in FIG. 10, and comprises the
following steps.
[0121] Firstly, a modulation scheme is selected, covering at least
two different modulation symbols, and preferably at least four
different modulation symbols.
[0122] Then, for the light phase modulating device of the present
type, respective fixed light phase shifts are selected, using the
said phase shifting means along the primary and secondary
waveguides of each child MZ as described above, so that destructive
interference results for each of said modulated symbols.
[0123] Then, modulation takes place of the symbols in said
modulation scheme, by selecting different variable light phase
shifts for said light phase modulating device along the said
primary and secondary waveguides of each child MZ.
[0124] It is preferred that the said modulation of said symbols is
performed in a push-pull fashion, as described above, using phase
shifts I+, I-, Q+ and Q-. In particular, it is preferred that the
modulation depth is not complete in this push-pull modulation.
[0125] In particular, it is preferred that the said modulation is a
phase shift keying or binary phase shift keying modulation
scheme.
[0126] Moreover, the invention also relates to a method for
continuously monitor a modulated optical signal using an optical
interference modulator of the type described herein. In particular,
this method is useful with modulators of the present type in which
reflected light exiting from a particular primary-end MMI coupler
access port is conveyed, via a waveguide, to a second output. Such
second output detectors are generally denoted "MPD" in the
Figures.
[0127] Such method is illustrated in FIG. 11, and comprises the
following steps.
[0128] Firstly, a light detector is provided, arranged to detect
said light conveyed to said second output MPD. This light detector
may be an integrated component on the same chip as the modulator in
question.
[0129] Then, the said light detector MPD is caused to detect an
amplitude and/or a phase of the detected light, and to report the
measurement value to the control unit, which in turn performs the
monitoring.
[0130] The continuous monitoring in question is preferably
performed in a feedback manner, so that the measurement value
report signal from said light detector to the control unit is used,
by the control unit, to modulate the phase shifting means of the
modulator in question in response to a change in said measured
amplitude and/or phase.
[0131] Above, preferred embodiments have been described. However,
it is apparent to the skilled person that many modifications can be
made to the disclosed embodiments without departing from the basic
idea of the invention.
[0132] A number of detailed examples have been presented. However,
the basic principles of the present invention in terms of using MMI
couplers as splitters and combiners in a modulator with folded
geometry are applicable to a broad spectrum of such modulators.
This is also the case for the basic idea of allowing a respective
second primary-side access port of such an MMI coupler feed light
to a modulator output, while a first primary-side access port feeds
light back to a modulator input. For instance, modulators may
comprise more, and more complex, sets of components, such as more
than two child MZI's, as long as the principles disclosed herein
are respected.
[0133] In such more complex approaches, asymmetric splitting and
combining can, for instance, be used.
[0134] In general, everything which is said in relation to one of
the said examples is freely applicable to other compatible
examples. Hence, the individual aspects of the present invention
have been presented in relation to specific examples, but are in
general more broadly applicable across various combinations of the
presented examples. In particular, many of the detailed features of
the solution according to the present invention have been presented
in relation to FIG. 2a, and they are, on an individual basis,
applicable to the other exemplifying embodiments presented in FIG.
6 and onwards, with one exception being the arrangement of the
third MMI coupler.
[0135] Modulation schemes useful with the present modulators and
methods comprise binary amplitude modulation with a return-top-zero
(RZ) or non-return-to-zero (NRZ) optical pulse stream format, and
phase shift keying modulation techniques, such as Binary Phase
Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), and
Quadrature Amplitude Modulation (QAM) techniques, such as QAM8,
QAM16 and QAM64. In each of these communication formats, the
modulated light wave will carry information about one or several
symbols selected among a predetermined set of symbols.
[0136] Hence, the invention is not limited to the described
embodiments, but can be varied within the scope of the enclosed
claims.
* * * * *